An antigen is any substance that causes your immune system to produce antibodies against it. This means your immune system does not recognize the substance, and is trying to fight it off.
An antigen may be a substance from the environment, such as chemicals, bacteria, viruses, or pollen. An antigen may also form inside the body.
Abbas AK, Lichtman AH, Pillai S. Antibodies and antigens. In: Abbas AK, Lichtman AH, Pillai S, eds. Cellular and Molecular Immunology. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 5.
Stedman's Online Medical Dictionary. www.stedmansonline.com/content.aspx?id=mirA1400011566&termtype=t. Accessed Sept. 9, 2015.